erbB3 is a newly identified receptor of erbB3/EGFR family with a still unknown function. The expression and the possible function of this receptor in normal and tumor cell lines is being investigated. It has been shown that erbB3 may have a possible role in gastrointestinal tumors. The presence and the activation, in terms of phosphorylation, of this receptor in several cell lines has been identified. Despite similarity with other members of the family (EGFR and erbB2), a different metabolic pathway for erbB3 using a chimeric receptor molecule in NIH/3T3 cells has been identified. to better understand the function of erbB3, a search for a specific ligand for this receptor was initiated. A protein secreted in the conditioned media of certain tumor cells capable of activating the erbB3 receptor was also identified. The N-terminal sequence of the purified protein revealed a structure distinct from that of other growth factors identified to date.